In semiconductor devices, particularly semiconductor devices having a multi-layered structure comprising a plurality of stacked thin films, not only the quality of each of the constituent thin films but also their interfacial states greatly influence their performances.
Therefore, in order to obtain a high performance semiconductor device having a multi-layered structure, it is extremely important to make each of the constituent semiconductor thin films of a high quality such that it has few defects or is free of defects. Particularly extra care is required when growing a high quality crystalline thin film on a single crystal substrate or a crystalline thin film by way of epitaxial growth. Various studies have been made to consider this situation.
The formation of a crystalline film by way of epitaxial growth in the prior art is generally performed by means of a CVD process. The CVD process to form, for example, a silicon crystalline thin film is usually carried out at a high temperature operation of 1000.degree. C. or above. In this case, there are various restrictions for the related conditions because of the high temperature. Additionally, it is difficult to satisfy the requirements of shallowing or/and steeply grading a dopant profile in the case of obtaining a highly integrated semiconductor device of a high performance. In view of this, in recent years, research and development have been centralized not on the CVD process but on MBE (molecular beam epitaxy)(see, for instance, A. Ishizaka and Y. Shiraki, J. Electrochem. Soc., 133, pp. 666 (1986)) and a technique of forming an epitaxial film at low temperature by a sputtering process or the like in which the film formation is performed in an ultra-clean atmosphere.
However, there are disadvantages as for the MBE process. The film formation process is required to be performed at an elevated temperature of at least 800.degree. C. in order to obtain a high quality Si epitaxial thin film and it is difficult to perform high concentration doping. Other than these disadvantages, there is the additional disadvantage that it is necessary to precisely and constantly maintain the liquid level of an evaporation source stored in a crucible at a prescribed temperature. Thus, this process is not always effective to form a high quality epitaxial film over a long period of time since the process control is relatively difficult as above described, and the size of the crucible used is limited.
As for the sputtering process, there are advantages such that the film formation may be performed at relatively low temperature, reproducibility is good, continuous film formation is possible, and the process is relatively easily controllable. However, there are inherent disadvantages for the sputtering process such as deposition of a reaction product caused by plasma reaction on the inner face of the reaction chamber, removal of the reaction product deposited, release of atmospheric component gas adsorbed at the inner wall face of the reaction chamber, and release of the constituent materials of the inner wall face of the reaction chamber as a result of the inner wall face being sputtered during film formation. These foreign matters contaminate the film-forming space, often resulting in their being incorporated into a film formed. These problems cannot be disregarded but they are required to be eliminated to obtain a high quality epitaxial film by the sputtering process. Thus, there is a demand for eliminating these problems.
In order to meet the above demand, there are proposed an apparatus comprising a deposition preventive member disposed in a film-forming chamber such that it circumscribes a plasma generation region including the space between a pair of electrodes, wherein the deposition preventive member serves to prevent foreign matters from depositing on the inner face of the film-forming chamber, and another apparatus of the same constitution as that of the former apparatus, except that the apparatus is designed such that a bias voltage can be applied to the deposition preventive member to prevent the constituent materials of the deposition preventive member from being sputtered out.
Specifically, Japanese Patent Laid-open No. 50962/1990 (hereinafter referred to as "Literature 1") discloses a film-forming apparatus using a sputtering process which comprises a film-forming chamber provided with a deposition preventive member therein and a vacuum chamber in which the deposition preventive member previously used is replaced by a new deposition preventive member. This film-forming apparatus is shown in FIG. 4. In FIG. 4, numeral reference 402 indicates a film-forming chamber for forming a prescribed film on a substrate 433. Numeral reference 400 indicates a deposition preventive member, and numeral reference 406 indicates a vacuum chamber in which the deposition preventive member previously used is replaced by a new deposition preventive member. Numeral reference 425 indicates an opening and closing means which is disposed between the film-forming chamber 402 and the vacuum chamber 406. The deposition preventive member 400 is transported from the vacuum chamber 406 to the film-forming chamber 402 or from the film-forming chamber to the vacuum chamber 406 by means of a transporting means 460. Numeral reference 440 indicates a target, numeral reference 401 indicates a shutter, and numeral references 411 and 412 respectively indicate an exhaust valve. In the vacuum chamber 406, there is disposed a baking means 450 which serves to release gas adsorbed at the surface of the deposition preventive member 400.
The film formation in the film-forming apparatus shown in FIG. 4 is carried out in the following manner. That is, the deposition preventive member 400 is subjected to baking treatment in the vacuum chamber 406, followed by transporting the deposition preventive member thus treated into the film-forming chamber 402 by means of the transporting means 460. Film formation is performed by a sputtering process in the film-forming chamber 402 to form a deposited film on a substrate 433 arranged in the film-forming chamber.
According to the film-forming apparatus shown in FIG. 4, film formation may be efficiently performed since deposition of the foregoing reaction product onto the inner face of the film-forming chamber 402 is prevented by the deposition preventive member 400 and the deposition preventive member 400 previously used can be replaced by a new deposition preventive member.
However, in using this apparatus, gases originating from the atmospheric component gas such as H.sub.2 O, CO.sub.2, O.sub.2, etc. remaining in the vacuum chamber 406 become adsorbed at the deposition preventive member 400 as the temperature of the deposition preventive member decreases, no matter how much the deposition preventive member had been subjected to baking treatment in the vacuum chamber. The adsorption of these gases at the deposition preventive member is unavoidably caused even in the case where the inside of the vacuum chamber 406 is evacuated to a high vacuum degree. This results in a problem occurring when the deposition preventive member 400 into the film-forming chamber 402 and film formation is performed therein so that the deposition preventive member 400 is heated not only because of thermal energy from plasma generated in the region circumscribed by the deposition preventive member 400 but also because of thermal energy caused upon heating the substrate. Whereby the gases adsorbed at the deposition preventive member 400 are released to contaminate the film-forming atmosphere. There is also another problem in this case. Charge particles in the plasma collide against the surface of the deposition preventive member 400 to sputter the constituent materials of the deposition preventive member 400 to release those materials, resulting in contaminating the film-forming atmosphere.
Further, The Institute of Electronics, Information and Communication Engineers SDM90-85 ("PRECISION CONTROL OF PLASMA PARAMETERS IN ADVANCED PLASMA PROCESSING EQUIPMENT" (Aug. 31, 1990), pp. 57-61) (hereinafter referred to as "Literature 2") reports a sputtering process capable of performing film formation while applying a voltage to a shield member (a deposition protective member) for the purpose of controlling the potential of plasma.
Literature 2 describes a suggestion that according to the process described therein, metal contamination caused as a result of the shield member being sputtered may be controlled.
However, Literature 2 describes only control of the plasma potential and indicates a mere possibility for the control of metal contamination caused as a result of the shield member being sputtered.
It is presumed that according to this film-forming process, a film of a good quality to certain extent could be formed since the film formation by a sputtering process is carried out in the film-forming space circumscribed by the shield member (the deposition protective member) while applying a bias voltage to the shield member to control the potential of plasma generated in the film-forming space. However, because gases originated from the atmospheric component gas are adsorbed at the shield member as well as the constituent member of the film-forming chamber, these gases are unavoidably released upon performing the film formation to contaminate the film formed.